E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The grapes are sour. You disparage it because it is beyond your reach. The allusion is to the well-known fable of the fox, which tried in vain to get at some grapes, but when he found they were beyond his reach went away saying, I see they are sour.
Wild grapes. What has been translated wild grapes (Isaiah v. 24) the Arabs call wolf-grapes. It is the fruit of the deadly nightshade, which is black and shining. This plant is very common in the vineyards of Palestine.